Definition of "half hour"

[U]pon the whole she spent her time comfortably enough; there were half hours of pleasant conversation with Charlotte, and the weather was so fine for the time of year, that she had often great enjoyment out of doors.

1813 January 27, [Jane Austen], chapter VII, in Pride and Prejudice: […], volume II, London: […] [George Sidney] for T[homas] Egerton, […], pages 80–81

Every half hour or so one or the other would steal off with snowshoes and toboggan to make the round of the holes, often returning with half a dozen fish that together weighed perhaps twelve pounds, perhaps twenty-four pounds; […]

1897, Charles A. Bramble, “Winter Fishing Through the Ice”, in The Sportsman’s Magazine‎, page 430

I was a night owl and could be a bit grumpy (mean, Michelle would say) within the first half hour or so of getting out of bed.

2006, Barack Obama, “Family”, in The Audacity of Hope: Thoughts on Reclaiming the American Dream, New York, N.Y.: Crown Publishers, page 338‎

Your stomach secretes ghrelin in pulses every half hour, sending subtle chemical impulses to your brain—almost like subliminal biological messages (carrot cake, carrot cake, carrot cake).

2006, Michael F[redric] Roizen, Mehmet C[engiz] Oz, with Ted Spiker, Lisa Oz, and Craig Wynett, You, on a Diet: The Owner’s Manual for Waist Management, New York, N.Y.: Free Press, page 44